This invention relates generally to capacitors, and more particularly to capacitors made with non-oxide electrodes and oxide dielectrics having high dielectric constants but with reduced leakage current, and to methods of making such capacitors and their incorporation into DRAM cells.
The increase in memory cell density in DRAMs presents semiconductor chip designers and manufacturers with the challenge of maintaining sufficient storage capacity while decreasing cell area. One way of increasing cell capacitance is through cell structure techniques, including three dimensional cell capacitors. The continuing drive to decrease size has also led to consideration of materials with higher dielectric constants for use in capacitors. Dielectric constant is a value characteristic of a material and is proportional to the amount of charge that can be stored in a material when the material is interposed between two electrodes. Promising dielectric materials include BaxSr(1-X)TiO3(xe2x80x9cBSTxe2x80x9d), BaTiO3, SrTiO3, PbTiO3, Pb(Zr,Ti)O3(xe2x80x9cPZTxe2x80x9d), (Pb,La)(Zr,Ti)O3 (xe2x80x9cPLZTxe2x80x9d), (Pb,La)TiO3(xe2x80x9cPLTxe2x80x9d), KNO3, Nb2O5, Ta2O5, and LiNbO3, all of which have high dielectric constants making them particularly desirable for use in capacitors. However, the use of these materials has been hampered by their incompatibility with current processing techniques and their leakage current characteristics. The leakage current characteristics of Ta2O5 on electrodes such as polysilicon, W, WNx, and TaN are not as good as those of the conventional silicon nitride capacitor.
Leakage current is controlled not only by the quality of the Ta2O5 dielectric film, but also by the state of the interface between the Ta2O5 film and the electrodes. Attempts have been made to overcome the problems associated with the use of Ta2O5. Some of the efforts have focused on post-Ta2O5 treatments, such as annealing in the presence of ultraviolet light and ozone (UV-O3 annealing), dry O2 annealing, or rapid thermal nitridation (RTN), to repair the oxygen vacancies in the as-deposited chemical vapor deposited (CVD) Ta2O5, film by excited oxygen or nitrogen atoms. Other work has focused on depositing special layers around the Ta2O5 film to prevent oxidation during later processing. For example, U.S. Pat. No. 5,768,248 to Schuegraf involves the deposition of a dielectric nitride layer after the removal of an oxide layer on a capacitor plate. A Ta2O5 dielectric layer is then deposited, followed by a second nitride layer. The nitride layer restricts oxidation of the inner capacitor plate during subsequent annealing of the Ta2O5 layer. In U.S. Pat. No. 5,814,852 to Sandhu et al., a primarily amorphous diffusion barrier layer is deposited on the Ta2O5 dielectric layer.
Chemical vapor deposited (CVD) Ta2O5 dielectric films are prepared in an oxygen gas mixture at elevated temperatures. Consequently, the bottom electrode in a capacitor stack, onto which the Ta2O5 film is deposited tends to be severely oxidized by the process. This results in a high leakage current, as well as low capacitance.
Non-oxide electrodes have been shown to be promising electrodes for use with high dielectric constant oxide dielectrics. However, the resulting leakage current is high for thinner films or layers of oxide dielectrics such as Ta2O5. Therefore, there is a need for improved processes for incorporating non-oxide electrodes, such as TiN, TaN, WN, and W, and high dielectric constant oxide dielectric materials such as Ta2O5 and BaxSr(1-x)TiO3, in capacitor constructions having improved leakage current and for capacitors containing these materials.
The present invention meets these needs by providing a stabilized capacitor using non-oxide electrodes and high dielectric constant oxide dielectric materials and methods of making such capacitors. By xe2x80x9cnon-oxidexe2x80x9d electrode, it is meant an electrically conductive material which does not contain any metal oxides. By xe2x80x9chigh dielectric constant oxide dielectricxe2x80x9d materials we mean oxides of aluminum, barium, titanium, strontium, lead, zirconium, lanthanum, and niobium, including, but not limited to Al2O3, BaxSr(1-X)TiO3(xe2x80x9cBSTxe2x80x9d), BaTiO3, SrTiO3, Ta2O5, Nb2O5, PbTiO3, Pb(Zr,Ti)O3(xe2x80x9cPZTxe2x80x9d), (Pb,La)(Zr,Ti)O3(xe2x80x9cPLZTxe2x80x9d), (Pb,La)TiO3(xe2x80x9cPLTxe2x80x9d), KNO3, and LiNbO3 and having a dielectric constant of at least about 20.
In accordance with one aspect of the present invention, the method includes providing a non-oxide electrode, oxidizing an upper surface of the non-oxide electrode, depositing a high dielectric constant oxide dielectric material on the oxidized surface of the non-oxide electrode, and depositing an upper layer electrode on the high dielectric constant oxide dielectric material.
The surface oxidation of the non-oxide electrode can be carried out in an atmosphere containing an oxidizing gas such as O2, O3, H2O, or N2O at a temperature in the range of from about 250xc2x0 to about 700xc2x0 C. The oxidation can be performed in the same reaction chamber as the step of depositing the high dielectric constant oxide dielectric material prior to depositing the high dielectric constant oxide dielectric material. Preferably, the oxidation is a gas plasma treatment which is carried out at a temperature in the range of from about 250xc2x0 to about 500xc2x0 C., although other oxidation techniques such as furnace oxidation or rapid thermal oxidation (RTO) may be used. The high dielectric constant oxide dielectric material is selected from the group consisting of Al2O3, BaxSr(1-X)TiO3, BaTiO3, SrTiO3, Ta2O5, Nb2O5, PbTiO3, Pb(Zr,Ti)O3, (Pb,La)(Zr,Ti)O3, (Pb,La)TiO3, KNO3, and LiNbO3, and preferably comprises either Ta2O5 or BaxSr(1-X)TiO3.
Another aspect of the invention is a capacitor which includes a non-oxide electrode, the upper surface of which is oxidized. The capacitor includes a high dielectric constant oxide dielectric material adjacent the upper surface of the non-oxide electrode, and an upper layer electrode adjacent the high dielectric constant oxide dielectric material. In a preferred embodiment, the non-oxide electrode is preferably selected from the group consisting of TiN, TaN, WN, and W, and the high dielectric constant oxide dielectric material is selected from Al2O3, Ta2O5 and BaxSr(1-X)TiO3. The upper surface of the non-oxide electrode is preferably oxidized using an oxidizing gas plasma such as O3.
Another aspect of the present invention is a DRAM cell and method of making it. In a preferred form, the method comprises providing a non-oxide electrode, oxidizing an upper surface of the non-oxide electrode, depositing a high dielectric constant oxide dielectric material on the non-oxide electrode, depositing an upper layer electrode on the layer of high dielectric constant oxide dielectric material, providing a field effect transistor having a pair of source/drain regions, electrically connecting one of said source/drain regions with the non-oxide electrode and electrically connecting the other of said source/drain regions with a bit line.